Oki OKIOFFICE44 Service Manual page 282

2) Fuser unit temperature control
The heater in the fuser unit is controlled by the thermistor, IOGA and CPU to keep the
heater roller surface within a prescribed temperature range. The CPU supervises the status
of PC0 (A/D converter input section) periodically, turning HEATON- N signal on and off
according to CPU of PC0 status to exercise temperature control.
At power on time, the CPU switches the output signal THON from pin 65 (between high
and low states) to check for a blown or shorted thermistor according to the status of the
THCHK signal.
A built-in thermostat in the fuser unit prevents the heater from being overheated in event of
failures in the thermistor, or temperature control circuit, etc.
Figure A3.1.16 shows the fuser unit temperature control.
Note: Heater control
2)-1 Heater control
The ACON control signal on the secondary side is transmitted to the primary side via PC3
to turn TRC1ON for AC voltage supply to the heater.
F1
ACIN
THCHK
5V
Board Descriptions
Temperature of the heater at the time of printing is 150 ˚C to 180 ˚C. This tempera-
ture is maintained by controlling the on and off operation of heater according to the
input of the thermistor converted into analogue-digital (A/D) values by the CPU.
048-POW
+5V
ACON
PC3
TRC1
AOUT
Thermostat
Heater
Thermistor
Fuser unit
Figure A3.1.16 Fuser Unit Temperature Control
A - 40
R44
HEAT-N
Heater
on/off circuit
CN7
0V
10
CN6
13
OLHV/TLHV
Board
OKIOFFICE 44 Serv. Man., P/N 59276801
IC2
HEATON
IOGA
CPU
THON
PA11
THCHK
PC0
0V